FUSE (Filesystem in Userspace) allows non-privileged users to create their own file systems. It works by mounting the file system within the userspace virtual file system. Python has a FUSE library called fusepy that provides a simple interface for implementing FUSE file systems in Python. PEPFS is an example of a FUSE file system implemented in Python that makes Python Enhancement Proposals (PEPs) available as read-only files organized in a file system structure. It uses fusepy and lazily downloads specific PEP files on demand when read.

1. Writing file system
in CPython
Dmitry Alimov
2018

2. File System
Application
Virtual File System (VFS)
Device Driver
Device (HDD, SSD, CD-ROM, NIC, etc)
User Space
Kernel Space Syscall Interface
Block Layer
Page CacheDirect I/O
Hardware
ext2, ext3,
ext4, btrfs
NFS,
smbfs
FUSE
procfs,
tmpfs

3. FUSE (File System In Userspace)
Available for Linux, FreeBSD, OpenBSD, NetBSD (as puffs), OpenSolaris, Minix 3,
Android and macOS [2]
In Linux kernel since version 2.6.14
Windows compatibility is provided by libraries and ports [3, 4, 5]

4. FUSE
Diagram showing how FUSE works [2]

5. Example uses
GlusterFS: Clustered Distributed Filesystem
GmailFS: Filesystem which stores data as mail in Gmail
SSHFS: Provides access to a remote filesystem through SSH
WikipediaFS: View and edit Wikipedia articles as if they were real files
πfs: A file system that stores all files in the digits of Pi

6. struct fuse_operations {
int (*getattr) (const char *path, struct stat *stbuf);
...
int (*readdir) (const char *path, void *buf,
fuse_fill_dir_t filler, off_t offset,
struct fuse_file_info *fi);
...
int (*read) (const char *path, char *buf, size_t size,
off_t offset, struct fuse_file_info *fi);
...
};
libfuse API

7. Python interface to FUSE
fusepy module [7] — simple interface to FUSE and MacFUSE:
def getattr(self, path, fh=None):
if path != '/':
raise FuseOSError(errno.ENOENT)
return {'st_mode': (S_IFDIR | 0o755), 'st_nlink': 2}
def readdir(self, path, fh=None):
return ['.', '..']
def read(self, path, size, offset, fh=None):
return self.data[path][offset:offset + size]

8. PEPFS
Read-only file system with PEPs as the files [8]
Uses github repository with PEPs to get the current PEPs [9]
Implemented in Python and uses the fusepy module
Lazy PEP files' read (download specific PEP on demand)

9. PEPFS mount
$ ./pepfs.py /tmp/pepfs/
$ mount
...
PEPFS on /tmp/pepfs type fuse
(rw,nosuid,nodev,relatime,user_id=1000,group_id=1000)
...

10. PEPFS example
$ ls -la /tmp/pepfs/
total 4
drwxr-xr-x 2 root root 6943251 Jul 25 00:07 .
drwxrwxrwt 35 root root 4096 Jul 25 00:07 ..
-rw-r--r-- 1 root root 29582 Jul 25 00:07 pep-0001.txt
-rw-r--r-- 1 root root 8214 Jul 25 00:07 pep-0002.txt
-rw-r--r-- 1 root root 2229 Jul 25 00:07 pep-0003.txt
...
-rw-r--r-- 1 root root 81947 Jul 25 00:07 pep-3333.txt

11. Page Cache
To enable Page Cache, you need to set the flag keep_cache, in open() method:
def open(self, path, flags):
flags.keep_cache = 1
return 0
And also set raw_fi to True in FUSE(PEPFS(), ..., raw_fi=True)
NB: Invalidation of cache and updating of data occurs only if the file size changes

12. Questions
https://t.me/spbpython
https://t.me/piterpy_meetup

13. References:
1. https://en.wikibooks.org/wiki/The_Linux_Kernel/Storage
2. https://en.wikipedia.org/wiki/Filesystem_in_Userspace
3. https://en.wikipedia.org/wiki/Dokan_Library
4. https://github.com/crossmeta/cxfuse
5. http://www.secfs.net/winfsp/
6. https://github.com/libfuse/libfuse
7. https://github.com/fusepy/fusepy
8. https://github.com/delimitry/pepfs
9. https://github.com/python/peps/